Since, the mobile number that has been given to you was a 0917-30 series, you can choose between the following series: 0917-30, 31, 32, 62, 63, 65, 67, 68, 70, 71, 72, 77, and 79. The mobile numbers are somewhat grouped, so if your number belongs to group 1, you can only select between the series on the same group if you plan to change your mob no.
The solution that I can see, as I have also experience this before is you go to a Business Center, ask for your sim card to be changed, the one that has the same group or hlr with the 0917-8s and 0917-5s. And by then, you can change the mobile number to 0917-8s and 0917-5s series.
Ideally, you can really choose any number you want, as long as it's within the series range, it's available on their new system and no one else is using or have used it.
Btw, change of mobile number can be also be made via their Cust Care hotline.
I did that exactly.
Greenbelt GBC terminated my line, changed my sim and gave me my preferred number.
What I lament with the experience that I had with them was the wasted time -- they made me wait for a month before I learned that the change of number was impossible in my case.
They could have provided me my preferred number upon application but they were giving me run around replies then that their new system could not, etc. only to do the exact thing they should have done a month.
Now, I'm thinking twice of getting another line with them.